Flexographic printing in India has
undergone a sea change in the last few decades. My own experience dates back to
the 1980s when as siliconiser I was selling silicone coated release paper
sheets to screen printers manufacturing stickers. I was under pressure to brand
my silicone sheets to get higher brand positioning, value and increase in
demand. Since the reverse of the silicon sheets was blank, few other suppliers
of release paper located in Mumbai started printing the same image of a diamond
to differentiate the uncoated side from the coated side so that sticker
manufacturers would know which side to use as a protective layer for the
pressure sensitive adhesive on the sticker. It was this need to print that lead
me to learn about flexographic printing process and print my own registered
trademark. It was a very rudimentary process whereby a butanol based ink was
picked up by a rubber roller and transferred through an intermediate roller to
a rubber plate mounted on another roller and the image was then in a moving web
transferred to the paper. The rubber printing plate was natural rubber based
and made more like a rubber stamp after vulcanizing the patterned sheet. In fact,
Ram Nagar in Paharganj area of Delhi was home to manufacturers of rubber stamps.
I located a vendor who would make the sheets for flexo printing for me to print
the backside of coated release paper by flexographic printing in a secondary
process after siliconising. Nowadays the same back printing of release liners is
done in line by roto gravure process while producing the silicone coated release
paper or filmic liners.
Developments in flexographic
printing process or Flexo as it is now referred to has made it a preferred printing
technology in recent time. It is now being considered almost at par with offset
printing and the last two decades of the new millennium 2000, have seen the
rapid adoption of this technology in PSA or Self Adhesive Label production or
narrow web printing may it be labels, shrink sleeves, lamitubes or flexible
packaging. Biggest advances in flexographic printing came about in the 1990s
with the development of photopolymer plates. These are synthetic plates made
with light sensitive polymers that harden and cross link at exposed areas in
design on exposure to light. Further developments in platemaking have made it
possible by computer aided etching the images and designs on the plates. The
factors that aided proliferation and use of flexo in label production include
the whole gamut of printing, embellishing and converting. These are as follows
and each process is a complete technology and warrants attending individually;
1. pre-press
2. plate
making
3. plate
mounting
4. Web
movement controls
5. Press
design
·
Inks
·
Ink delivery system/Aniloxes and their cleaning system/doctor
blades
·
Print head construction
·
Hot air/UV/LED UV
·
Web path
·
Tension controls
·
Auto Registration
·
Online inspection
·
Turn bar
·
Delam/Relam
·
Lamination/cold foiling/Hot foiling
·
Embossing/ die-cutting/ slitting/rewinding
·
Waste matrix handling
·
Higher production speeds
·
Other specialized processes
·
Automatic butt splicers/reel changers to achieve
24X7 production
·
Hybrid/combination
Plates:
Developments
in prepress and plate making has provided the biggest impetus to the
improvements in flexo and its implementation in label production. Most of the
important development in flexographic printing is attributed to photopolymer
printing plates, including improvements to the plate material and the method of
plate creation. Digital computer to plate systems has done well for the
industry. Companies like Asahi Photoproducts, AV Flexologic, Dupont, MacDermid,
Kodak and Esko have pioneered the latest technologies, with advances in fast
washout and the latest screening technology. In India, the credit for upgrading
and bringing flexo printing to a quality almost similar to that of offset, goes
largely to Dupont and Esko. A good plate can ensure good printing if and only
if the plate has been mounted on the print cylinders accurately. Flexographic
plate mounting is a critical step in achieving optimum print quality. Using correct
mounting tape, proper techniques in preparation, mounting and plate removal
will allow the optimum performance of the mounting tape through the entire
flexographic printing process. From the manual mounting of plates in the 1990s
we now have reached a level where automatic plate mounting machines are
available for achieving perfection in print quality and registration. Mounting
tapes play a crucial role in flexo. High print
quality at high press speeds can be directly associated with the quality of the
foam used in the plate mounting tapes, a wrong plate mounting tape can impact
print quality adversely. Specialized soft plate mounting tapes that act
as a shock absorber as they compress and return to the same thickness are now
available. The tape adhesive is also an important part, these adhesives have
been developed such that they stick firm to avoid edge lifting and also be
cleanly removable for the reusability of plates. Plate thickness and tape
thickness are matters of importance as they impact the circumference and repeatability,
but technical support is now easily available on these parameters from the
suppliers.
Inks
and ink delivery:
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| UV Curing image: courtesy Labelsandlabeling |
Flexo
liquid inks have also undergone a lot of developments in the intervening
period. From solvent based to water based hot air drying to UV cured and then LED
UV cured inks. The evolving of the process into UV ink has brought in an era of
high-speed printing and reproducibility with accuracy. With water based flexo the color matching of
each job (even when it was a repeat) used to be a challenge due to ever
changing viscosity of the ink lying in store shelf or in the pan while running
on the press or due to changing temperatures it affected the print quality. Availability
of UV inks in the late 1990s and full UV flexo presses changed the flexo
printing scenario. Subsequently operators could easily operate machines, handle
the inks without any difficulty eliminating the need for measuring ink
viscosity, ink pH, color variation, setting time to get the exact color with reduced
wastages. Anilox rolls play an important role of measured ink delivery system. Anilox
rollers are cylinders highly engineered metal and/or ceramic roll that are
laser engraved with minute cells of a given angle, line screen, and volume to
deliver the required amount of ink to achieve fineness of print. Higher line
count aniloxes enable printers to go to higher plate screen line count, fine print
and reduced ink consumption with higher speeds. Laser-etched ceramic anilox
rolls also play an important part in the improvement of print quality due to
their ability to control Ink volume transfer making full-color picture printing
possible Further developments in introduction of LED UV is resulting in reduced
energy consumption. The subject of print quality, flexo dot size, dot gain, distortion
etc. and their impact on colours or tones is a full subject that we shall
discuss separately in another writeup.
Printing station:
The printhead has had an
interesting journey from the simple set of rollers driven by gears as described
in my opening paragraph to a highly technical workstation delivering print quality
that matches the best produced. The quality of gears improved over the years,
the line shaft was accurately designed, the number of gears reduced to reduce
any visible gear marks, the whole cluster was re-engineered to achieve finesse
and facilitate quick change overs, automatic registration controls with cameras
or sensors and finally introduction of servo drives. The servo drive eliminated
the gears, helped in reduced wastages, achieve better speeds and excellent
registration control in both machine and cross direction in automation. The advanced print impression
pre-settings, a computer control pad, register pre-settings and job storage
data improve the flexo printing experience. This drastically reduced dependence
on operators. The anilox placement was redesigned for easy lift off for replacement.
The simple ink tray, doctor blade which is an important part of flexo printing
process is now replaced in seconds after introduction of Chamber doctor blade
which is a recent development in the flexographic inking process. It is an
enclosed chamber doctor blade system in which two doctor blades are used along
with an enclosed chamber to dispense the ink to the anilox roll. it enables
printing with smaller quantity of ink without contamination from atmosphere and
faster changeover. The whole ink cartridge system and anilox of printing head can
be changed in seconds enabling fast changeover of jobs.
Drying System:
Hot air drying required a long
web path and by the time an operator could reach the level of acceptable
registration he would end up with over a 100 running meters of wastage
impacting the cost. Ever since 2007 when short web path was introduced along
with installation of UV lamps to dry and cross link backed with chill drums aided
by fully automatic registration control systems with cameras at each station was
introduced, the amount of wastages has come down substantially. On the other
hand, the development of UV LED flexo inks give faster production speeds
compared to traditionally employed UV curing. It also offers longer life
of lamps, instant on or off capability, reduced maintenance, and faster
changeovers.
Finishing:
Aiding better quality e.g.
to eliminate pinholes when a solid ground is to be printed web cleaners are installed
to have a cleaner web eliminating micronic dust particles or fibers that may
have flown of the blades during slitting
and settled on the web. Corona treaters are now a standard equipment that aid
the anchorage of the ink and coatings to the substrates. Inline embellishing
and finishing have brought innovation into printing, decorating and converting of
labels. These include cold foiling, lamination, hot foiling, turn bar for front/back
printing all on movable rails to position at any station. An important part of
the finishing in self-adhesive labels is die cutting and waste matrix removal.
These could be any printer’s nightmare, especially when producing special
shapes. A change of die with a change of magnetic cylinder meant anything
between half an hour to one hour using crane to lift and replace the heavy
magnetic cylinder. In today’s time when time is money when we consider it as an
input in our costing, one cannot imagine such loss of time. Machine design have
been modified by press manufacturers such that now one can just slide out and
slide in the new cylinder in a matter of seconds. Complex die cut labels result
in waste matrix that cannot withstand the rigors of highspeed converting. Most machine
manufacturers have redesigned their waste matrix removal innovatively to keep performing
efficiently at full machine speeds. Other processes that are added as required
include inspection, embossing, debossing, slitting and rewinding all inline in
a single process.
Machine design and
engineering is an extremely important part of machine building. The precision
in developing and creating each moving part such that there is no unwanted vibration
or shiver when the machine speed is increased or decreased. Some machines may
have all the features that are there in a high-end press, but it runs well
until say 70-80 meters per minute while internationally acclaimed label presses
run at 200 meters per minute. For price considerations some printers are
tempted to opt for the former, not realizing that in the long run the more
expensive, high speed and stable machine returns a better Return on Investment
(ROI). It is thus advisable to dwell on each aspect of the machine, its setup, consistent
quality output, speed and maintenance. Capital Investment is not like buying
raw material repeatedly and where you can change your supplier whenever desired
but capital equipment is a one-time investment which has a definite impact on
your company’s future.
Written by Harveer Sahni, Chairman Weldon Celloplast
Limited, New Delhi October 2020
Printing magazine are permitted to reproduce the above article by giving due credit to author including the blog address http://harveersahni.blogspot.com
